1. Technical Field
The present invention relates to an ultrasonic cleaning apparatus and an ultrasonic cleaning method.
2. Description of the Related Art
FIG. 4 is a cross-sectional view schematically showing a conventional ultrasonic cleaning apparatus.
The ultrasonic cleaning apparatus has a liquid storage part 112, and an ultrasonic vibrator 111 that applies ultrasonic vibrations to a cleaning liquid 113 in the liquid storage part 112. A contact surface 111a on which the ultrasonic vibrator 111 comes into contact with the cleaning liquid 113 is directed downward. Namely, the contact surface 111a faces the direction in which the force of gravity is applied (the direction in which the cleaning liquid 113 falls).
Furthermore, the ultrasonic cleaning apparatus has a cleaning liquid supply mechanism 116 that supplies the cleaning liquid 113 to the liquid storage part 112. The cleaning liquid supply mechanism 116 has a supply pipe 115, a flow control mechanism 114 and a cleaning liquid supply source (not illustrated).
Next, a method for cleaning an object to be cleaned such as a substrate through the use of the ultrasonic cleaning apparatus will be explained.
First, a cleaning liquid 113 such as water or a chemical liquid is supplied to the supply pipe 115 by the cleaning liquid supply source, and the cleaning liquid whose flow rate has been controlled by the flow control mechanism 114 is supplied to the liquid storage part 112 through the supply pipe 115 and is temporarily stored in the liquid storage part 112. Ultrasonic vibrations are applied to the cleaning liquid 113 stored in the liquid storage part 112, through the use of the ultrasonic vibrator 111, and the cleaning liquid 113 to which the ultrasonic vibration has been applied is discharged to the outside of the liquid storage part 112 through a nozzle 121. The cleaning liquid 113 discharged maintains a state where ultrasonic vibrations have been applied, and is supplied to an object to be cleaned (not illustrated), which is subjected to ultrasonic cleaning (see, for example, Japanese Patent Laid-open Publication No. 2007-289807).
Incidentally, there is a request for reducing the amount of the cleaning liquid that is used in cleaning an object to be cleaned. If the amount of the cleaning liquid can be reduced, cleaning costs can be reduced. In addition, there is a case of performing cleaning through the use of a very expensive cleaning liquid, and in the case of the cleaning liquid, the effect of reducing cleaning costs is large.
However, in the above-described conventional ultrasonic cleaning apparatus, there is a limit to reduction in the use amount of the cleaning liquid, and the amount of the cleaning liquid used cannot sufficiently be reduced. The reason thereof will be explained below.
In the above-described ultrasonic cleaning apparatus, if the amount of the cleaning liquid 113 to be supplied to the liquid storage part 112 by the cleaning liquid supply mechanism 116 is excessively reduced, a state in which the cleaning liquid 113 is not filled in the liquid storage part 112 is brought about, and a part that does not come into contact with the cleaning liquid 113 (a cavity) is generated in the contact surface 111a of the ultrasonic vibrator 111 to thereby be put into a water-free operation state. The water-free operation leads to excessive amplitude of the ultrasonic vibrator 111 and causes a failure of the ultrasonic vibrator 111. Therefore, the amount of the cleaning liquid 113 used cannot be reduced exceeding a level that will induce the water-free operation state.
As a method for further reducing the use amount of the cleaning liquid 113, it is considered that the amount of the cleaning liquid 113 to be discharged from the nozzle 121 is reduced by making the diameter of the nozzle 121 smaller. However, if the diameter of the nozzle 121 is made too small, ultrasonic waves are attenuated when the cleaning liquid to which ultrasonic vibrations have been applied passes through the nozzle 121, and thus the ultrasonic waves cannot pass through the nozzle 121 and the lowering of the cleaning effect is caused. Therefore, it is necessary to set the diameter of the nozzle 121 to be larger than the wavelength of the ultrasonic waves in order not to attenuate ultrasonic waves.
Specifically, when ultrasonic vibrations of 430 kHz in frequency are applied to the cleaning liquid 113, the wavelength of the ultrasonic wave is represented by sonic speed/frequency=1500/430000, which is about 3.5 mm. Furthermore, when an ultrasonic vibration of 950 kHz in frequency is applied to the cleaning liquid 113, the wavelength of the ultrasonic wave is represented by sonic speed/frequency=1500/950000, which is about 1.6 mm. Accordingly, desirably, the diameter of the nozzle 121 is set to be 3.5 mm or more when the frequency of ultrasonic waves is 430 kHz, or the diameter of the nozzle 121 is set to be 1.6 mm or more when the frequency of ultrasonic waves is 950 kHz. As described above, the diameter of the nozzle 121 cannot be reduced only to the extent of not attenuating ultrasonic waves, and the use amount of the cleaning liquid cannot sufficiently be reduced.